Search for neutral heavy leptons produced in ZZ decays

Weak isosinglet Neutral Heavy Leptons (νm) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived νm production giving monojet or acollinear jet topologies, and for long-lived νm giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 → νmν̄) of about 1.3 × 10-6 at 95% confidence level for νm masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the νm mass. The results are also interpreted in terms of limits for the single production of excited neutrinos. © Springer-Verlag 1997

Determination of |Vcb_{cb}| from the semileptonic decay B0^{0} --> D∗−*^{-}l+ν^{+}\nu

Semileptonic decays B \rightarrow D^{*-} \ell^+ \nu X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant %(D^{**}) and non-resonant D^{*-} \pi^{+} states was performed and the ratio of the branching fractions: \frac {Br ({\mathrm{B} \rightarrow {\mathrm D}^{*-}} \ell^+ \nu X)} {Br({\mathrm{B} \rightarrow {\mathrm D}^{*-}} \ell^+ \nu X) + Br( {\mathrm{B}^0 \rightarrow \mathrm{D}^{*-}} \ell^+ \nu)} = 0.19 \pm 0.10({\mathrm{stat}}) \pm 0.06({\mathrm{syst}}) was determined. Taking into account this contribution, %DB the differential production fraction of the decay the differential decay width of {\mathrm{B}^{0} \rightarrow \mathrm{D}^{*-}} \ell^+ \nu was measured as a function of the momentum transfer from the B to the D^{*-} in two separate analyses, using exclusive and inclusive methods of D^{*-} reconstruction. The distributions were fitted %DB with a linear function over the full momentum transfer range to extract the product of \mathrm{|V_{cb}|} times the normalization of the decay form factor F(q^2_{max}): F(q^2_{max})\mathrm{|V_{cb}|} = (35.0 \pm 1.9({\mathrm{stat}}) \pm 2.3({\mathrm{syst}}) )~\cdot~10^{-3}. The value of \mathrm|V_{cb}| was computed using theoretical calculations of F(q^2_{max}), giving: {\mathrm{|V_{cb}|}} = (38.5 \pm 2.1({\mathrm{stat}}) \pm 2.5({\mathrm{syst}}) \pm 1.7({\mathrm{theory}}))~\cdot~10^{-3}. The total branching fraction Br(\mathrm{B}^{0} \rightarrow D^{*-} \ell^+ \nu) was determined to be: Br ({\mathrm{B}^0 \rightarrow \mathrm{D}^{*-}} \ell^+ \nu) = (5.47 \pm 0.16({\mathrm{stat}}) \pm 0.67({\mathrm{syst}})) \%

Study of rare b decays with the DELPHI detector at LEP

Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from 1991 to 1994. These decays are mediated by both tree level $b \rightarrow u$ and one-loop penguin $b \rightarrow s$, $d$ transitions. Evidence for charmless $B$ decays was obtained in two body hadronic modes. The branching ratios of $B^{0}_{d,s}$ to $\pi^+ \pi^-$ or $K^+ \pi^-$ and $B^{-}_{u}$ to $\rho^0 \pi^-$ or $K^{*0} \pi^-$ were found to be $(2.8 ^{+1.5}_{-1.0} \pm 0.2) \times 10^{-5}$ and $(1.7 ^{+1.2}_{-0.8} \pm 0.2) \times 10^{-4}$ respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectator $s$ quark, was measured to be $0.58 \pm 0.18$. Upper limits were set at 90\% confidence level on the branching ratios %for other two body modes including the $\Lambda_b^0\to pK^-$ decay and for three and four body charmless hadronic decays in the range of \mbox{$(1 - 3)\times10^{-4}$}, for inclusive radiative $b \rightarrow s \gamma$ decays at $5.4\times10^{-4}$, for the exclusive radiative decays $B_{d}^{0} \rightarrow K^*(892)^0\gamma$ and $B_{s}^{0} \rightarrow \phi(1020) \gamma$ at $2.1\times10^{-4}$ and $7.0\times10^{-4}$ respectively, and for dineutrino decays, $b \rightarrow s \nu \bar \nu$, in the exclusive channels $B_{d}^{0} \rightarrow K^*(892)^0 \nu \bar \nu$ and $B_{s}^{0} \rightarrow \phi(1020) \nu \bar \nu$ at 1.0 $\times$ 10$^{-3}$ and 5.4 $\times$ 10$^{-3}$ respectively. The limits on dineutrino decays constrain theories with a new $U(1)$ gauge boson coupling predominantly to the third family of fermions

Study of radiative leptonic events with hard photons and search for excited charged leptons at root s=130-136 GeV

During the last 1995 data acquisition period at LEP, the DELPHI experiment collected an integrated luminosity of 5.9 pb(-1) at centre-of-mass energies of 130 GeV and 136 GeV. Radiative leptonic events (e, mu, tau) with high energy photons were studied and compared to Standard Model predictions. The data were used to search for charged excited leptons decaying through an electromagnetic transition. No significant signal was found. From the search for pair produced excited leptons, the limits m(e)* > 62.5 GeV/c(2), m(mu)* > 62.6 GeV/c(2) and m(tau)* > 62.2 GeV/c(2) at 95% confidence level were established. For single excited lepton production, upper limits on the ratio lambda/m(l)* of the coupling of the excited charged lepton to its mass were derived

First study of the interference between initial and final state radiation at the Z resonance

The interference between initial and final state radiation in the process e(+)e(-) --> mu(+)mu(-) at root s approximate to M(Z) has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using the KORALZ generator, with and without O(alpha) interference, The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution. Using the theoretical predictions, a value of Gamma(Z) = 2.50 +/- 0.21 (stat.) +/- 0.06 (syst.) GeV. has been extracted, The interpretation of this result is discussed. There is an additional uncertainty in the estimate of Gamma(Z) from as yet uncalculated higher order interference terms. By assuming a value of Gamma(Z) consistent with the world average, the data were used to estimate the size of these uncalculated corrections